Metal-on-metal (MoM) bearings for hip replacement have surged in popularity over the last several years due to lower volumetric wear (compared to conventional metal-on-ultrahigh molecular weight polyethylene) and the ability to use large femoral heads, reducing the risk of dislocation, a common complication of hip replacement necessitating revision surgery. Large femoral heads also provide the surgeon with the option of resurfacing the femoral head without breaching the femoral metaphysis or diaphysis (hip resurfacing arthroplasty) thereby preserving femoral bone stock. However, a cause for concern with MoM joint reconstruction, has recently emerged with increasing reports of early adverse local tissue responses compromising the functionality and survivorship of these reconstructions. These adverse responses, which include periprosthetic bone loss (osteolysis), delayed-type hypersensitivity and soft tissue masses (so-called pseudotumors) are governed by the local cellular reaction to particulate and ionic wear and corrosion debris. There is growing evidence that the local cell response is related to the amount of debris generated by these bearing couples. Thus, there is an urgent clinical need to delineate the mechanisms of debris generation in order to minimize these adverse local tissue responses. Unfortunately, the tribology of this bearing couple is little understood. Our laboratory has made the novel observation that metal-on-metal bearings undergo microstructural changes and tribochemical reactions with the joint environment during articulation. This behavior causes changes in the metallurgy of the upper surface and brings about the generation of a mechanically mixed zone of nanocrystalline metal (oxide) and organic constituents. Such a zone - also called tribomaterial - favorably influences the tribological properties as well as the tribocorrosive behavior, as has been shown for related boundary lubricated applications in mechanical and automotive engineering. In the present proposal, we will address the central hypothesis that the combination of metal particles and denatured proteins form a biotribological layer in metal on metal bearings that has synergistic effects in reducing wear and corrosion. We have assembled a multidisciplinary investigative team that will address the following specific aims that will likely have a significant impact of the performance of MoM bearings in hip replacement: 1) to determine the role of the biotribological layer constituents in the evolution of the near-surface region of the metal using wear simulations for accelerated testing and compare these to device retrievals obtained from patients undergoing MoM hip or surface replacement;2) to determine the relevant properties of the mechanically mixed zone of retrievals and in-vitro samples and demonstrate the effect of embedded organic constituents;and 3) to demonstrate that the presence of a mechanically mixed zone will beneficially effect the (tribo-)corrosion behavior of the cobalt-chromium alloy